The development of the mammalian central nervous system (CNS) begins in the early stage of embryonic development and continues until the post-natal period. The first step in neural development is cell birth, which is the precise temporal and spatial sequence in which neural stem cells and neural stem cell progeny (i.e. daughter neural stem cells and neural progenitor cells) proliferate. Proliferating cells give rise to neuroblasts, glioblasts, and new neural stem cells. The next step is a period of neural cell differentiation and migration, which give rise to the neurons and glial cells that migrate to their final positions. The neural component of the mature mammalian CNS is composed of neuronal cells (neurons) and glial cells (astrocytes and oligodendrocytes).
In mammals, specialized glial cells called radial glia develop immediately before the differentiation and migration of neurons. These radial glial cells span the cerebral wall from the ventricular surface of the neuroepithelium to the pial surface, forming a scaffolding for the initiation and maintenance of neuronal cell migration. Through a series of reciprocal signaling events between the migrating neurons and the radial glia, neurons migrate from their site of origin to their final position along the elongated processes of the radial glial cells. During neuronal migration, radial glial cells do not divide. After neuronal production and migration end, however, the radial glia enter a mitotic cycle, eventually differentiating into multipolar astrocytes. In lower vertebrates, radial glia has the capacity to form neurons but it is currently unclear whether radial glia or other types of glial precursors have the same capacity in mammals. Collectively, several studies suggest that may be only a small (and often reversible) transition between neuroepithelial stem cells and radial glia.
The use of cells for neural transplantation is well documented. Several studies have indicated that primary tissue from the developing ventral mesencephalon can give rise to dopaminergic neurons and supporting cells capable of survival, function, and therapeutic efficacy in Parkinson's patients. In addition, the transplantation of cultures containing neural precursor cells and stem cells can give rise all three major cell subtypes of the CNS, i.e. neurons, astrocytes, and oligodendrocytes. From these studies, there is a clear need in the art for cells capable of proliferating to make large numbers of cells as well as a capacity for neural differentiation in order to make the appropriate “adult” cells capable of integrating and restoring function to a diseased area in the CNS. Furthermore, over the past couple of decades, protein factors capable of protecting neural cells in the CNS from damage and capable of restoring function have bee discovered. From neuroprotection studies, it is evident that these protein factors may best work if delivered by gene manipulated cells placed in the area of disease. Thus, there is also a need in the art for transplantable neural cell lines capable of being gene modified in order to secrete protein factors locally. In addition, cell lines capable of making neurons and other neural lineages in a reproducible manner are useful screening targets to identify factors and drugs capable of influencing the CNS. Hence, there is a need in the art for neural cell lines for drug screening purposes. Last, with the human genome almost completely sequenced, there is a need for cells of neural lineages, which can be used to identify cDNA libraries to screen for gene function.
If glial precursor cells of the mammalian CNS could form neurons, astrocytes and perhaps other subtypes, a dividing pool of glial precursor cells could become a reliable source of large numbers of neural cells for the needs described above identifying several areas of industrial application. Preferably, cellular division in such glial precursor cells would be epigenetically regulated, so that a suitable number of glial precursor cells could be efficiently prepared in sufficient numbers for transplantation. However, these cells could also be genetically modified in order to be made to proliferate or differentiate in a reproducible manner. Furthermore, the cells could be genetically modified in order to produce a protein factor suitable as a therapeutic. The unmodified or gene modified glial precursor cells should be suitable in autografts, xenografts, and allografts as well as for in vitro use to screen for drug activity or gene expression. Protocols allowing for stable and long-term propagation of glial precursor cells would therefore be of great value. If such cultures could grow over extended periods, their properties would be interesting to compare to those of neural stem cells.